Ultrasonic, flow measuring devices are often applied in process- and automation technology. They permit in a simple manner, the determination of volume flow and/or mass flow in a pipeline. Known ultrasonic, flow measuring devices usually work according to the Doppler principle or the travel-time difference principle.
In the case of the travel-time difference principle, the different travel times of ultrasonic pulses are evaluated relative to the flow direction of the liquid. For this, ultrasonic pulses are transmitted at a certain angle to the tube axis both with as well as also counter to the flow. From the travel-time difference, the flow velocity, and therewith, in the case of known diameter of the pipeline section, the volume flow can be determined.
The ultrasonic waves are produced, and, respectively, received, with the assistance of ultrasonic transducers. Travel time is ascertained in U.S. Pat. No. 5,052,230 by means of short, ultrasonic pulses.
Normally, ultrasonic transducers are composed of an electromechanical transducer element, e.g. a piezoelectric element, and a membrane, or diaphragm. Ultrasonic waves are produced in the electromechanical transducer element and, in the case of clamp-on-systems, led via the membrane, or also coupling layer, to the tube wall and from there into the liquid. In the case of inline systems, the ultrasonic waves are coupled into the measured medium via the membrane. In such case, the membrane is also called an ultrasound window.
Between the piezoelectric element and coupling layer or ultrasound window, another layer can be arranged, a so called adapting, or matching, layer. The adapting, or matching, layer performs, in such case, the function of transmission of the ultrasonic signal and simultaneously the reduction of reflections caused by different acoustic impedances at interfaces between two materials.
Used as adapting, or matching, layer is an isotropic material, usually a resin or an adhesive, in order, supplementally to the transmission, to apply the electromechanical transducer element fixedly to the ultrasound window, as disclosed in DE 10 2008 055 123 B3. WO 2009/024403 A1 teaches, moreover, a technology, whereby the thickness of the adapting, or matching, layer can be predetermined.
DE 10 2007 042 663 A1 discloses that also an adapting, or matching, layer of oil is known; compare DE 295 09 574 U1. However, an adapting, or matching, layer both made of adhesive, as well as also one of oil, have unique disadvantages. Through different thermal expansions of ultrasound window, adhesive and electromechanical transducer element, frequently mechanical stresses are experienced in the named components, which can lead to inability of the ultrasonic transducer to function well. Oil tends, for example, to out-gas in the case of higher temperatures, or it loses its contact providing effect in the case of low temperatures.